1. Field of the Invention
The present invention relates to a ballpoint pen, and more particularly, to a ballpoint pen accommodating ballpoint oil-based ink in an ink-accommodation cylinder.
2. Description of Related Art
A ballpoint pen has a ballpoint pen tip in which a ball is rotatably held. A conventionally known oil-based ink for such a ballpoint pen contains coloring agent, organic solvent, resin or the like, and its ink viscosity is in a range of 10000 to 30000 mPa·s (20° C.).
According to such conventional ballpoint oil-based ink, the ink viscosity is set to 10000 mPa·s or higher, a film is formed between the ball and a ball seat to reduce metal contact resistance between the ball and the ball seat, or lubricant or the like is used to enhance the lubricity, the contact resistance between the ball and the ball seat is reduced, and wear of the ball seat is suppressed. However, there is a problem that the lubricity is insufficient in a long-distance writing especially under high pressure, the ball seat is worn and writing performance is deteriorated.
In the conventional ballpoint pen accommodating the ballpoint oil-based ink, an amount of ink consumed when writing through 100 m is about 10 mg when its ball diameter on the tip end ob the ballpoint pen is φ0.5 mm, about 20 mg when the ball diameter is φ0.7 mm, and about 30 mg when the ball diameter is φ1.0 mm, and the amount of ink consumed is about 0.23 mm when the writing width is φ0.5 mm, about 0.28 when the writing width is φ0.7 mm and about 0.33 mm when the writing width is φ1.0 mm.
It seems possible to increase the writing density by increasing the consumption of ink, but the writing density is not increased merely by increasing the consumption of ink. This is because that the writing density and an ink amount per unit area have an intimate relation, and even if the ink consumption is the same, if the writing width is increased due to blur, the writing density is lowered.
According to the oil ballpoint pen, Z is usually smaller than 1.0×10−3 wherein the ink consumption is Y mg, an amount of ink per 1 mm2 is Z mg/mm2, and Y=X×1.0×105×Z. The conventional ballpoint pen is inferior in the writing density.
The present inventors studied the conventional oil ballpoint pen, and found that if Z is equal to or greater than 1.0×10−3 (wherein writing width is X, the ink consumption per 100 m is Y and the ink amount per 1 mm2 is Z), the writing density became higher than the conventional writing density and a written line became clear, and the inventors achieved the present invention.
To enhance the condition of the pen, it is conceived to lower the ink viscosity, but if the ink viscosity is merely lowered, although the condition of the pen is enhanced, since the film thickness between the ball and the ball seat becomes thin, there is a problem that the metal contact resistance between the ball and the ball seat can not be lowered, the ball seat is worn and the writing performance is deteriorated.
To solve the problems, it is well known to add lubricant into ballpoint oil-based ink composition to enhance the lubricant between the ball on the ballpoint pen tip and a tip body.
As ink for enhancing the lubricant, there is proposed ballpoint ink in which organic acid, non-alkali metallic salt of naphthenic acid, lanolin derivative, fluorine-based and/or silicone-based surfactant or nonyl phenol-based phosphoric ester is added as described in Japanese Patent Application Laid-open No. S52-123722 “ink”, Japanese Patent Application Laid-open No. S 55-84376 “ballpoint pen ink”, Japanese Patent Publication No. H1-39467 “ballpoint pen ink”, and Japanese Patent Application Laid-open No. H11-293174 “oil-based ink composition and oil ballpoint pen”.
Japanese Patent Application Laid-open No. H6-212111 “ballpoint oil-based ink composition” proposes a ballpoint oil-based ink composition which contains oleic acid and aliphatic amine, enhances lubricant and has little time degradation.
However, if the conventional organic acid, non-alkali metallic salt naphthenic acid, lanolin derivative, fluorine-based and/or silicone-based surfactant or nonyl phenol-based phosphoric ester is only used as the lubricant, there is a problem that such lubricant is not satisfactory as a ballpoint pen having lubricity under high writing pressure and excellent writing comfort, and stability is deteriorated with time.
If oleic acid and aliphatic amine are contained as in patent document 5, although time degradation can be suppressed, lubricity can not be maintained under high writing pressure and the writing comfort is not excellent.